This invention relates generally to aircraft radio systems, and more particularly to an adapter, making it possible for a pilot to hear well with ordinary earphones without interfering with concurrent loudspeaker operation, the adapter also functioning as an intercom within the aircraft.
The flow of aircraft traffic is mostly in an airway pattern that is aligned with ground-based, very high-frequency omnidirectional radio ranges and distance-measuring systems. These navigational facilities supply the pilots of equipped aircraft with information on the bearing to or from the ground station and the distance thereto. Navigational systems are also employed for en route and instrument approach purposes, the navigation function being normally performed in the aircraft by the pilot or navigator.
Flight information as well as advisory, weather, flight-plan revisions and control information are handled by high-frequency voice communication transceiver channels between the plane and air-route traffic-control centers, airport towers and air-traffic communications stations.
The main concern of the present invention is with navigation-communications equipment of the type installed in small private planes in general aviation, usually propeller-driven, as distinguished from equipment for large passenger planes, generally of the jet type, used in commercial aviation.
In a private plane, it is the usual practice to feed the audio output of the navigation-communication system into a loudspeaker, so that all incoming intelligence may be heard by the pilot and all other occupants of the plane, including in some instances a co-pilot or navigator. In a typical private plane, there is little acoustic insulation and the ambient noise level is extraordinarily high, for one is subjected not only to the loud whirring of the propellers and the engine noises but also to wind sounds.
The output of the loudspeaker is in direct competition with such ambient noise. Even with the speaker volume turned up to override ambient noise, it is still quite difficult for the pilot and other occupants of the plane to discriminate between intelligence issuing from the loud speaker and noise.
Nevertheless, it is not the present custom for pilots to listen on earphones to the navigation-communication system. With properly padded earphones, one obtains a marked reduction of ambient sound so that the pilot is then able to clearly hear the incoming intelligence, more or less to the exclusion of ambient noise. But ambient noise is not entirely a distraction to be fully excluded, for it is important for the pilot to listen to such noise so that he can quickly detect irregularities in engine performance and take the necessary corrective measures.
Even though the pilot experiences difficulty in hearing intelligence over the loudspeaker and has available an earphone jack on his equipment, he rarely uses earphones. One reason why pilots have heretofore found the use of earphones objectionable despite their obvious advantages over a loudspeaker in a cockpit environment is the earphones specially designed to operate with the installed system are expensive and are not usually usable in another installation.
An uninterrupted speaker operation is essential so that all occupants of the plane have the benefit of the communicated intelligence; hence one does not disconnect the speaker when listening on the phones. But in the conventional aircraft system, the earphones and the loudspeaker both impose a load on the same audio output, so that if the volume control for the system is set to a level suitable to the pilot listening on the phones, it will not result in a satisfactory operating level for the loudspeaker, and vice-versa.
Because each aircraft navigation-communication system is designed for a particular type of head-set, with existing arrangements the pilot cannot interchangeably use a head-set whose impedance is intended to match the output impedance of a given aircraft system with the audio output of another system. Nor is it possible to use an inexpensive head-set adapted say for use with a stereo phonograph amplifier, with an aircraft system. These factors have discouraged the use of head-sets.
As a consequence, most general aviation pilots forego the use of earphones and the advantages to be gained therefrom, and try, as best they can, to extract information from the loudspeaker. Inasmuch as there are situations where the communication of intelligence may be vital to safety, the inability to hear clearly is a serious drawback and may endanger the lives of a pilot and his passengers.
Another drawback of existing communication systems is that the pilot in a small plane has no means of communicating with a co-pilot or a passenger, save by shouting to be heard over the ambient noise. This can be exhausting to the pilot, particularly if he is seeking to instruct a student pilot.